A linear image sensor such as a charge-coupled device (CCD) is a solid-state imaging device which performs a photoelectric conversion on a one-dimensional optical image incident thereon and outputs an electrical signal generated by the photoelectric conversion in time series. An image sensor of the related art is described in, for example, Patent Literatures 1 to 3. Among such image sensors, a linear image sensor (one-dimensional CCD) is used for a different purpose from a two-dimensional image sensor, and different characteristics are required.
FIG. 18 shows a linear image sensor of the related art. A plurality of pixels are aligned in a row direction and there is only one pixel in a column direction in a light-sensitive region. Charges generated in each pixel are transferred to a horizontal register through a region directly beneath a transfer gate electrode according to incidence of an energy beam. The charges input to the horizontal register are transferred in a horizontal direction, and are output to the outside through an amplifier.
Such a linear image sensor is used in a spectroscopic analysis to detect weak light. In order to realize a high S/N ratio, it is preferable to increase a light-receiving unit area of one pixel to increase the number of signal photons incident on the one pixel. In this case, a size of a photoelectric conversion region of one pixel in a length direction (charge transfer direction) is increased.
In a CCD, generated charges are transferred using a potential inclination referred to as a fringing electric field, but when a pixel size is large, a potential becomes flat with respect to a position at a center of the pixel, and charges tend to be difficult to transfer. Therefore, in a linear image sensor having a large-sized photoelectric conversion region in a length direction, a resistive gate electrode is disposed on the photoelectric conversion region through an insulation film, and a potential inclination is intentionally formed in the photoelectric conversion region.
On the other hand, in a linear image sensor for emission spectral analysis (OES) including laser-induced breakdown analysis (LIBS), a high-speed shutter function has been required. The spectroscopic analysis can be performed by detecting a spectroscopic optical image using the linear image sensor.
In the emission spectral analysis, a distribution and a state of elements contained in an inspected object can be identified by emitting strong excitation light (laser and the like) to the inspected object and performing the spectroscopic analysis on a bright-line spectrum emitted from the inspected object.